Technical Field. The present invention relates to a method of reducing emissions in the exhaust gases of an internal combustion engine having at least one cylinder supplied with an air/fuel mixture when a crankshaft of the internal combustion engine rotates, at least one inlet valve, at least one inlet duct connected to the inlet valve, at least one exhaust valve, control members for controlling the opening and closing of the inlet and exhaust valves, and a piston reciprocating between a top dead-center position and a bottom dead-center position in the cylinder.
Background Information. It is desirable to reduce noxious emissions present in the exhaust gases of an internal combustion engine in order to reduce pollution of the surrounding environment and to satisfy legal requirements for internal combustion engines. The undesirable emissions present in the exhaust gases include, inter alia, carbon monoxide (xe2x80x9cCOxe2x80x9d), hydrocarbon compounds (xe2x80x9cHCxe2x80x9d) and nitrogen oxides (xe2x80x9cNOxxe2x80x9d).
In order to reduce these emissions in the exhaust gases, the engine is provided with a catalytic converter that, by means of a chemical reaction, burns the above mentioned emissions completely. This chemical reaction occurs in the catalytic converter only when the catalytic converter has reached a predetermined working temperature, which is reached after a predetermined operating time of the engine. Accordingly, during cold-starting of the engine there is no reduction of the above mentioned emissions in the catalytic converter.
There are known arrangements for heating the catalytic converter during cold starts in order to rapidly reach a desirable working temperature of the catalytic converter, making it possible to reduce the emissions in the exhaust gases of the engine at an early stage. In such a known arrangement, an electric heating element is arranged in the catalytic converter. However, this arrangement makes the catalytic converter complicated and expensive to produce.
Another problem that occurs when internal combustion engines are cold-started is a comparatively great amount of fuel in relation to the air supplied, or a rich air/fuel mixture, has to be supplied to the engine in order for the engine to start and be capable of working at an essentially constant speed when idling. This rich air/fuel mixture is also supplied so that the engine can provide increased torque when the accelerator is operated, and so that the engine is less sensitive to different fuel qualities. The drivability of the engine is thereby ensured before the engine has reached its operating temperature.
Without emission control of the catalytic converter and the rich air/fuel mixture, the content of CO, HC and NOx emitted from the engine is consequently high when the engine is cold-started.
Previous attempts have been made to reduce the quantity of fuel in relation to the air supplied, i.e., run the engine with a leaner air/fuel mixture when the engine is cold-started. These attempts have resulted in both the engine working very unevenly when idling and poor drivability of the engine. The engine speed varies while idling because the torque generated by the engine is very sensitive to variations in a lambda value of the air/fuel mixture supplied to the cylinder space of the engine when the air/fuel mixture is lean. The definition of the lambda value, or the excess air factor as it is also known, is the actual air quantity supplied divided by the air quantity theoretically necessary for complete combustion. If the lambda value is greater than 1, the air/fuel mixture is lean and, if the lambda value is less than 1, the air/fuel mixture is rich.
The fuel supplied from a fuel injection valve can be controlled accurately by means of the fuel injection system of the engine in order to obtain a substantially constant lambda value for the air/fuel mixture supplied. However, when the engine is cold, fuel condenses on the comparatively cold walls in the inlet duct and in the cylinder. The fuel condensed on the walls vaporizes and accompanies the air/fuel mixture flowing in the inlet duct and supplied to the cylinder space. If the vaporization of the fuel condensed on the walls is uneven, for example, due to pressure variations, temperature gradients, or the flow rate of the air/fuel mixture in the inlet duct, the lambda value of the air/fuel mixture supplied to the cylinder space will vary.
As the torque generated by the engine varies while idling when cold-started, the speed of the engine varies. In this connection, the speed of the engine refers to the speed of rotation of the engine crankshaft. When the speed varies, the pressure in the inlet duct also varies, resulting in the vaporization of the condensed fuel varying, so that a variation of the lambda value of the air/fuel mixture supplied to the cylinder space occurs. This intensifies the uneven speed of the engine.
The present invention provides a method of reducing carbon monoxide CO, hydrocarbon compounds HC and nitrogen oxides NOx in the exhaust gases of an internal combustion engine when cold-started.
The invention further provides a method for an internal combustion engine to work with a lean air/fuel mixture.
With the present invention, the working temperature of the catalytic converter can be rapidly reached. Additionally, the working temperature of the internal combustion engine can be rapidly reached.
This is accomplished by a method including the steps of supplying an air/fuel mixture with a lambda value greater than 1 to the cylinder, controlling the internal combustion engine so that it works at high load, and controlling the inlet valve so that it opens after the piston has passed the top dead-center position.
By supplying an air/fuel mixture greater than 1 to the cylinder, the total amount of emissions in the exhaust gases emitted from the internal combustion engine is reduced. In order to make it possible to operate the engine with a lean air/fuel mixture, the inlet valve is opened after the piston has passed the top dead-center position, thereby obtaining a powerful swirling of the air/fuel mixture supplied to the cylinder space. By controlling the engine so that it works at high load, condensed fuel on the walls of the inlet duct will have little effect on the mixing ratio between the air and the fuel, resulting in the lambda value of the air/fuel mixture supplied to the cylinder space remaining substantially constant. The crankshaft will thus rotate at a substantially constant speed when idling. Furthermore, the amount of residual gases are minimized as there is no overlap when the inlet and exhaust valves are open.